1. Field of the Invention
This invention is concerned with analysis of semiconductor processing solutions, particularly with determination of silicon concentration in silicon wafer etchant solutions.
2. Description of the Related Art
Etching processes are critical to fabrication of both circuitry and semiconductor devices on silicon integrated circuit (IC) chips. In one process, a silicon nitride (Si3N4) mask on a layer of silicon dioxide (SiO2) is patterned etched to expose the underlying silicon/silicon dioxide layer, which is then locally oxidized at high temperature (800-1200° C.) to produce thicker insulating SiO2 in unmasked areas to electrically isolate subsequently formed MOS (metal oxide semiconductor) transistors. The Si3N4 mask can withstand the high temperature but requires a strong etchant operated at high temperature (>150° C.). The silicon nitride etching process must be closely controlled to provide complete removal of the silicon nitride mask material without excessive etching of the underlying silicon dioxide layer. In particular, it is important to control the etch rate of silicon nitride relative to that of silicon dioxide.
The silicon nitride etchant is generally a concentrated solution of phosphoric acid (85 wt. %) and is operated at a temperature above 150° C. (typically 165° C.). The etch rate of silicon nitride and the selectivity with respect to silicon dioxide in this etchant solution depend strongly on the concentration of silicon ions, which are derived from the etching process and accumulate in the etchant solution with use. Silicon ions reduce the etch rates of both silicon nitride and silicon dioxide in the phosphoric acid etchant but tend to improve the selectivity. It is important that the change in the etch rates and selectivity resulting from accumulation of silicon ions be taken into account to optimize the silicon nitride etching process but available methods for determining the silicon concentration in concentrated phosphoric acid solution are inadequate.
Conventional methods of determining the silicon concentration in aqueous solutions involve reaction of silicon ions with ammonium molybdate to form the ammonium silicomolybdate salt, which is a yellow solid. This reaction is the basis for measuring silicon concentration by a variety of approaches, including those based on gravimetric, spectroscopic, electrochemical and ion chromatography methods. However, ammonium molybdate also reacts with phosphate ions to form an analogous compound that interferes with silicon concentration determinations based on ammonium silicomolybdate. Such interference precludes use of methods based on the ammonium silicomolybdate salt to determine the silicon concentration in silicon nitride etchant solutions containing high concentrations of phosphoric acid.
More sophisticated methods, based on atomic absorption analysis or inductively coupled plasma-atomic emission spectroscopy, for example, are available for analysis of silicon in concentrated phosphoric acid solution. However, such methods require equipment that is large, complex, expensive and costly to maintain, and are not amenable to automation and on-line use.
As described in U.S. Pat. No. 7,351,349 to Shekel et al. (issued 1 Apr. 2008), near infrared (NIR) spectroscopy may be used to detect fluoride species in some silicon dioxide etchant, surface preparation and cleaning solutions. However, available NIR spectroscopic methods and devices do not provide sufficient sensitivity for analysis of small concentrations of silicon in silicon nitride etchant solutions.
European Patent Application No. EP 1724824 A2 to Watatsu et al. (filed 12 May 2006) describes a method for analysis of silicon concentration in silicon nitride etchant solutions comprising concentrated phosphoric acid. In this method, HF added as concentrated hydrofluoric acid to the hot phosphoric acid etchant solution reacts with the silicon to form gaseous SiF4, which is hydrolyzed and detected via a change in conductivity of an aqueous solution. This method is cumbersome and time consuming, involves handling a hazardous gas (SiF4) and is not readily amenable to automation.
There is a need for an effective method of measuring low concentrations of silicon in silicon nitride etchant solutions so that the silicon nitride etch rate and selectivity can be controlled to improve quality and yield of semiconductor IC chips. Preferably, the method would provide accurate results within a short time frame using inexpensive equipment, and would be amenable to automation and on-line process control. Environmental impact of the method is also an important consideration.